The present invention relates to acoustics, more particularly to methods and systems for using acoustic sensors or acoustic sensor information to ascertain characteristics of sources of sound.
An acoustic “scalar” sensor measures a scalar component of an acoustic field, such as pressure. As distinguished from an acoustic scalar sensor, an acoustic “vector” sensor measures a vector (non-scalar) component of an acoustic field, such as particle velocity. A typical acoustic vector sensor includes both a scalar component (e.g., pressure) and a vector component (e.g., particle velocity) of an acoustic field. More specifically, a typical underwater acoustic vector sensor combines a tri-axial arrangement of motion-sensing devices (such as accelerometers or other wave/particle-velocity sensors) with a pressure-sensing hydrophone, in a neutrally buoyant package smaller than half a wavelength; see M. J. Berliner, J. F. Lindberg, Acoustic Particle Velocity Sensors: Design, Performance and Applications, AIP, Woodbury, N.Y., 1996, incorporated herein by reference. The vector sensors are used alone or in arrays to detect and localize sources of sound; see G. L. D'Spain, W. S. Hodgkiss, G. L. Edmonds, “Energetics of the Deep Ocean's Infrasonic Sound Field,” J. Acoust. Soc. Am., Volume 89, Number 3, pages 1134-1158 (March 1991), incorporated herein by reference; V. A. Shchurov, A. V. Shchurov, “Noise Immunity of a Combined Hydroacoustic Receiver,” Acoustical Physics, Volume 48, Number 1, pages 98-106 (January 2002), incorporated herein by reference; Benjamin A. Cray, “Acoustic Vector Sensing Sonar System,” U.S. Pat. No. 5,930,201, issue date 27 Jul. 1999, incorporated herein by reference.
Analytical models of vector sensor measurement systems have been developed to evaluate their detection performance (see B. A. Cray, A. H. Nuttall, “Directivity Factors for Linear Arrays of Velocity Sensors,” J. Acoust. Soc. Am., Volume 110, Number 1, pages 324-331 (July 2001), incorporated herein by reference) and their localization performance (see A. Nehorai, E. Paldi, “Acoustic Vector-Sensor Array Processing,” IEEE Trans. Sig. Proc., Volume 42, Number 9, pages 2481-2491 (September 1994), incorporated herein by reference; M. Hawkes, A. Nehorai, “Acoustic Vector-Sensor Beamforming and Capon Direction Estimation,” IEEE Trans. Sig. Proc., Volume 46, Number 9, pages 2291-2304 (September 1998), incorporated herein by reference). Generally speaking, acoustic sensor systems that effect conventional signal processing and implement vector sensors afford better sensitivity and resolution than do similar systems that implement scalar sensors.
Scalar modal beam processing was recently introduced as a processing scheme for spherical arrays of microphones (see J. Meyer, G. Elko, “A Highly Scalable Microphone Array Based on an Orthonormal Decomposition of the Soundfield,” ICASSP (13-17 May 2002), pages II-1781 to II-1784, incorporated herein by reference) and for circular arrays of microphones (see H. Teutsch, W. Kellermann, “Acoustic Source Detection and Localization Based on Wavefield Decomposition Using Circular Microphone Arrays,” J. Acoust. Soc. Am., Volume 120, Number 5, pages 2724-2736 (November 2006), incorporated herein by reference; H. Teutsch, W. Kellermann, “EB-ESPIRIT: 2D Localization of Multiple Wideband Acoustic Sources Using Eigen-Beams,” ICASSP (18-23 March 2005), pages III-89 to III-92, incorporated herein by reference). Rather than directly beamforming an array of signals, basically the following two-step process is used according to the aforementioned scalar modal beam processing: First, spherical or cylindrical modal beams are formed by suitably weighted sums of signals from a scalar sensor array. Second, the modal beams are then combined to form one or more computationally steerable directive beams. Scalar modal beam processing is limitedly effective, however, because the number of modal beams that can be formed from scalar array data is restricted to a few low-order modes by the number of sensors in the array, by the radius of the array, and by large differences in sensitivities of the computed mode; see J. Meyer et al., supra.
Non-linear beam-forming schemes have recently been reported; see J. A. Clark, G. Tarasek, “Localization of Radiating Sources along the Hull of a Submarine Using a Vector Sensor Array,” Oceans '06, IEEE, Boston, Mass., 18-21 Sep. 2006, incorporated herein by reference; K. B. Smith, A. V. van Leijen, “Steering Vector Sensor Array Elements with Linear Cardioids and Non-Linear Hippioids,” J. Acoust. Soc. Am., Volume 122, Number 1, pages 370-377 (July 2007), incorporated herein by reference; Dehua Huang et al., “Nonlinear Techniques for Pressure Vector Acoustic Sensor Array Synthesis,” U.S. Pat. No. 7,274,622 B1, issue date 25 Sep. 2007, incorporated herein by reference. Non-linear processing methods can further improve resolution; however, calibration is difficult of the output of measurement systems employing non-linear processing methods. Therefore, the use of non-linear processing methodology is often limited to qualitative indications of the sound field characteristics.